Drilling of oil wells has progressed from crude drilling rigs, to cable tool rigs, to the modern rotary drilling rigs. In rotary conventional drilling, a power rotating means delivers torque to a drill string comprising sections of drill pipe which turns a bit drilling a borehole into the subsurface formations. The drill string is raised and lowered in the borehole from support means affixed to a conventional drilling rig. Suspended over pulleys positioned at the upper end or top of the rig are a plurality of cables which support a traveling block. Suspended from the traveling block is a swivel. The swivel is secured to a kelly which supports the drill string. The kelly is square or hexagonal in cross-section over a substantial portion of its length and fits in sliding relation through a rotary table in the rig floor. The rotary table, driven by a suitable prime mover, serves to turn the kelly, thereby rotating the drill string. Due to the sliding fit between the kelly and the rotary table, the kelly slides downwardly through the rotary table as drilling progresses. While the power for rotating the kelly, and thus the drill string, is applied to the rotary table, the entire weight of the kelly and drill string is supported by the swivel which also functions to conduct drilling fluid to the kelly and drill string. Drilling fluid, generally from a mud tank or mud pit, passes through a hose into the swivel, downward through the sections of drill pipe, and out through openings in the drill bit into the borehole. The drilling fluid then circulates upward from the drill bit, carrying formation cuttings through the annulus between the sections of drill pipe and the borehole wall to the surface of the earth where it returns to the mud tank or pit. When it is necessary to add another section of drill pipe during drilling to the wellbore or to remove a section of drill pipe when pulling out of the borehole (i.e., tripping), the traveling block, swivel, and kelly are lowered or raised as needed by manipulation of the cables. Such a conventional drilling system is illustrated in U.S. Pat. Nos. 3,235,014; 3,324,717; 3,417,830; and 4,114,435.
Recent developments in drilling technology have replaced the conventional kelly and rotary table drive system with a power swivel employing an electric drive system for directly rotating the drill string. The power swivel is suspended from the traveling block and is fully compatible with the derricks or masts of the conventional drilling rig as well as the hoisting and electrical power systems of such rigs. One of the several advantages of the power-swivel top drive drilling system over the kelly and rotary table drilling system is the ability to rotate the drill string and circulate the drilling fluid when raising or lowering (i.e., tripping) of the drill pipe in or out of the borehole. This ability to rotate and circulate at any time while tripping provides significant time savings and safety features, especially where the potential for preventing sticking of drill pipe in tight sections or high angle boreholes is greatly increased.